It'll be so enormous, it's bound to have at the very least 8 hours of content. How do I know? Because this guy from The GamersHub spent 1 hour with the game at PAX and saw like 1/8 of it. More or less.

I didn’t get to see a chance of the famous Vegas Strip, but I did get a chance to run around the eerily accurate Mojave Desert. The one thing I will point out is that the world is enormous I probably was able to only see about an eighth of the world and I spent a good hour playing the game. My exploitations led me to a few firefights and here I used the games new crosshair shooting mode. The new viewing mode was needless to say not helpful, for me at least. Unless you’re using a sniper rifle the rifles do not do enough damage for the crosshairs to aid you at all times. I started using the cowboy repeater and using the new updated VATS system. There is not much new to the actual system so why they call it new is weird to me. The animations for the VATS are still as gory as ever so no need to worry about that.

Also, he had fun:

Some of the most fun I have ever had within a Fallout game happened at the Primm Casino. How often do you get to a high point in Fallout 3 and rain down grenades onto an evil bunch of dudes? Not often unless you count going to the top of the Washington Monument and just throwing stuff off of there, but that’s probably not a good idea still.

I wonder how many Fallout games he's actually played. Probably just the only true Fallout game - Fallout 3. Let's hope Obsidian will be able to prove themselves worthy of Bethesda's legacy.